a) Field of the Invention
The present invention relates to a liquid crystal display element and a method of manufacturing the same, and more particularly to such an element and method capable of improving an angle subtended by the field of view.
b) Description of the Related Art
In a liquid crystal display element or liquid crystal cell used by a liquid crystal display or the like, the orientation of liquid crystal molecules is changed to a different orientation by externally applying an electric field or the like. A change of this optical property is used to change a visual observation provided by the display element in order to provide a particular orientation of liquid crystal molecules under no electric field, an orientation process is generally performed on surfaces of glass substrates which hold the liquid crystal layer.
As a conventional orientation process for a twisted nematic (TN) liquid crystal cell, a so-called rubbing process has been used in which glass substrates holding a liquid crystal layer are rubbed by cotton cloth or the like.
For example, upper and lower substrates are rubbed in two orthogonal directions. For a negative display of a liquid crystal cell, polarizers of parallel configuration are disposed outside of the cell in such a manner that one of the rubbing directions aligns with the polarization axes of the polarizers. For a positive display, polarizers of crossed configuration are disposed in such a manner that the polarization axes become in parallel or perpendicular to the orthogonal rubbing directions.
The orientation of liquid crystal molecules becomes uniform by the rubbing orientation process. As a result, an angle of field of view is limited to a particular range within which the user can observe a display screen clearly.
FIG. 4A shows an example of equi-contrast curves indicating the characteristic of an angle of field of view of a TN phase liquid crystal cell. In FIG. 4A, the normal line direction of a liquid crystal cell is represented by .THETA.=0, wherein the angle .THETA. represents the radial angle relative to the normal line. The observing position in the horizontal plane is represented by an azimuth angle .phi.. The definitions of these angles are illustrated in FIG. 4B.
Thick solid curves in FIG. 4A are equi-contrast curves which are given contrast values. As seen from FIG. 4A, an angle of Field of view with a high contrast is limited to a particular range. Such a liquid crystal cell has a dependency on an angle of field of view wherein one direction provides a clear observation and another direction provides an unclear observation.
If the liquid crystal cell having the characteristic of an angle of field of view shown in FIG. 4A is used in a display device, the contrast lowers extremely at some angle relative to the display screen (in the example shown in FIG. 4A, near at .phi.=180 degrees). In a radical case, contact inversion occurs on the display screen.
The characteristic of an angle of field of view such as shown in FIG. 4A results from pre-tilts, shown in FIG. 5, of liquid crystal molecules caused by rubbing. The direction of pre-tilts of liquid crystal molecules, when projected on the substrate, coincides with the rubbing vector direction indicated by an arrow line in FIG. 5.
When a voltage is applied to a liquid crystal cell, liquid crystal molecules rise along the pre-tilt direction. Therefore, the optical rotary power tends to vanish when viewed from the pre-tilt direction on the mid-plane in the cell. The direction providing the clearest observation is therefore in this direction.
During rubbing, static electricity may be generated by friction, resulting in a possible insulation breakdown of an orientation film, or a deflective orientation and poor display. In the case of a liquid crystal cell of an active matrix (AM) drive type having drive elements such as thin film transistors (TFT), and wirings formed on the surface of the cell, static electricity caused by rubbing may destroy elements and wirings. Also in the case of a simple matrix type, fine wiring may be broken down or destroyed.
A great, amount of fine dusts are produced during the orientation film Forming or rubbing. Dusts attached to the substrate by static electricity may cause display defects such as an insufficient gap, black and white dots on the display cell.